1. Field of the Invention
The present invention relates to a process for preparing dialkyl carbonates by reaction of carbon monoxide (CO) with alkyl nitrites in the presence of a catalyst selected from the group consisting of platinum metal halides on a support of metal phosphate having acid centres.
Dialkyl carbonates are of general chemical importance. For instance, diethyl carbonate is an excellent solvent in the middle boiling-point range. Furthermore, dialkyl carbonates are excellent carbonylation and acylation reagents. Finally, they have great importance in the preparation of other carbonates, of urethanes and of ureas.
2. Description of the Related Art
It is known that dialkyl carbonates can be prepared by reactions of phosgene or of alkyl chloroformates with alcohols. However, there is increasing interest in avoiding the use of the toxic phosgene or the intermediates derived therefrom, such as the chloroformic esters, by means of other processes. Besides attempts to obtain dialkyl carbonates by reaction of CO with lower alcohols, processes in which CO is reacted in the gas phase with alkyl nitrite over a platinum metal catalyst are of particular importance. In such reactions, the formation of dialkyl oxalate in addition to the desired dialkyl carbonate is frequently observed.
Thus, the Journal for Catalytic Research (China), Volume 10(1), pp. 75-78, describes the reaction of CO and methyl nitrite over a PdCl.sub.2 -containing activated-carbon catalyst, which in addition to dimethyl oxalate gives mostly dimethyl carbonate.
A Pd/carbon catalyst for the preparation of dimethyl carbonate from CO and methyl nitrite is also described in Chin. Sci. Bull. 34 (1989), 875-76.
Furthermore, EP 425 197 discloses a process which in its preferred embodiment gives dialkyl carbonates of methanol or ethanol from CO and methyl or ethyl nitrite in the gas phase over a PdCl.sub.2 catalyst on activated carbon. According to said EP 425 197, Table 1, the selectivities in respect of the desired lower dialkyl carbonates reach values of up to 94%, but lower dialkyl oxalates and CO.sub.2 are always observed as by-products. Furthermore, the reported high selectivities were not satisfactorily reproducible when the work was repeated. The catalysts of this EP 425 197 contain additions of chlorides of base metals; a considerable amount of hydrogen chloride, namely an amount from 1 to 50 mol%, based on the platinum metal in the catalyst, is added to the system, or part of the catalyst has to be taken from the reactor and subjected to treatment with hydrogen chloride.
EP 501 507 describes the use of zeolites as support material. This does avoid some of the abovedescribed disadvantages, but at the same time a significant deterioration occurs in the selectivity in respect of the methyl nitrite used. Thus, according to Example 1, only a selectivity of 79%, based on methyl nitrite, is achieved.
In EP 503 091 and EP 503 618, the properties of the catalysts based on activated carbon are improved by further additions, for example, of copper and molybdenum or copper, molybdenum and potassium fluoride. But here too there occurs a decrease in catalyst activity, which in an industrial process leads to appreciable extra cost. These resulting disadvantages caused, such as increased need for regulation due to changing (falling) conversions and downtimes during replacement of the deactivated catalyst, are particularly noticeable in a cycle process in which the NO formed in the reaction is recirculated and converted into methyl nitrite. Also disadvantageous are the low conversions of methyl nitrite which are achieved. This is likewise disadvantageous in connection with the abovementioned cycle gas process.
In DE-OS (German Published Specification) 41 23 603, a high selectivity, based on both CO and methyl nitrite, with simultaneously high conversion is achieved by use of a palladium chloride catalyst with .gamma.-Al.sub.2 O.sub.3 as support material. However, maintenance of the catalytic activity requires the addition to the starting mixture of hydrogen chloride gas in amounts of up to 1000 ppm. This can lead to corrosion problems.